Compositions and methods for blackening hardened steel

ABSTRACT

Improved compositions and methods particularly adapted for the blackening of hardened steels. The present compositions consist essentially of conventional blackening compositions to which a small amount by weight of molybdic acid or salt is added.

United States Patent Mitchell Aug. 12, 1975 [54] COMPOSITIONS AND IVIETHODS FOR 2,557,507 6/1951 Miller 148/624 BLACKENING HARDENED STEEL 2,679,475 5/1954 Singlet 148/614 R 2,960,420 11/1960 Mitchell 148/624 [75] Inventor: Abraham J. Mitchell, Stratford,

Conn.

Primary Examiner-Ralph S. Kendall [73] Asslgnee' 2;: y g fg gsgj Chemical Attorney, Agent, or FirmThomas L. Tully; Arthur A.

0 Johnson [22] Filed: Oct. 24, 1974 [21] Appl. No.: 517,485

[57] ABSTRACT g 148/624; Improved compositions and methods particularly [58] g i R 6 15 R adapted for the blackening of hardened steels. The

"""" 7 6 present compositions consist essentially of conventional blackening compositions to which a small 5 References Cited amount by weight of molybdic acid or salt is added.

UNITED STATES PATENTS 8/1929 Pacz l48/6.l4R

9 Claims, No Drawings COMPOSITIONS AND METHODS FOR BLACKENING HARDENED STEEL The present invention represents an improvement over the compositions and methods of my earlier US. Pat. No. 2,960,420 relating to the blackening of ferrous metals. While the compositions and methods of my said patent are useful and effective for the blackening of unhardened steel, they produce unsatisfactory results in some cases and for certain purposes with respect to the blackening of hardened steels. Thus the blackening process is slow in the case of hardened steel and in many instances the deposit formed on the hardened steel is greyish or whitish-grey and completely unacceptable for certain uses or purposes.

Steel is hardened by a process in which the steel is heat treated to a critical elevated temperature and then quenched or cooled at a rate which prevents the formation of ferrite and pearlite and results in the formation of martensite. The cooling may be effected through the use of water, oil or air, depending upon the composition of the steel and its dimensions. Martensite is the hardest and major constituent of hardened steel and represents a solid solution of ferric carbide in very fine alpha iron. It is metallic greyish in color.

l have found that hardened steel is difficult to blacken because of the inertness of the hard carbide crystals present at the surface. The elemental iron surrounding the crystals at the surface can be blackened by the compositions of my aforesaid patent but the carbide crystals are chemically inert with respect to the ingredients of such compositions. Therefore only a portion of the steel surface is blackened and the steel has a dark greyish appearance rather than a black color.

It is the principal object of the present invention to provide a novel improved composition for the blackening of iron and steel, including stainless steel, and most particularly for the blackening of hardened steels more quickly and more effectively than heretofore possible.

It is an advantage of the present invention that the effective life of blackening baths based upon the present compositions is extended over the effective life of blackening baths based upon the compositions of my aforementioned US. patent.

These and other objects and advantages of the present invention will be apparent to those skilled in the art in the light of the present disclosure.

The improved compositions of the present invention are produced by the incorporation of from about 0.001 part by weight up to about 20 parts, most preferably from 0.1 to 5 parts, by weight of molybdic acid or an alkali metal molybdate salt to a conventional blackening composition consisting essentially of from 50 to 90 parts, most preferably from 60 to 80 parts, by weight of an alkali metal hydroxide, from 0.] to 50 parts, most preferably from 5 to parts, by weight of an alkali metal nitrate, from 0.1 to 50 parts, most preferably from 15 to 30 parts, by weight of an alkali metal nitrite, from 0.001 to parts, most preferably from 0.1 m5 parts, by weight of a cyclic sulphur-containing organic compound from the group consisting of Z-mercaptobenzothiazole, benzothiazyl disulphide and 2- mercaptoimidazoline, from 0 to 20 parts, most preferably from 0.001 to 1.0 parts, by weight of a wetting agent and from 0 to 20 parts by weight of a metal salt of a sulfonated dye.

l have found that the addition of the molybdic acid or salt to a conventional alkaline blackening bath containing both nitrates and nitrites and the cyclic sulphur compound provides an aqueous blackening bath which not only oxidizes the elemental iron present at the surface of hardened steel to form black iron oxide but also deposits black molybdic oxide over the surface of the carbide crystals, whereby the entire surface of the hardened steel is blackened in a minimum amount of time. The basic blackening composition, including the nitrates and nitrites, exerts a chemical reaction upon the elemental iron causing it to be oxidized to form black iron oxide, while the cyclic sulphur compound prevents the formation of reddish smut as disclosed by my aforementioned patent. The molybdic acid or salt, in the presence of both the nitrate and the nitrite, undergoes reaction and deposits a black molybdic oxide over the surface of the hardened steel including the surface of the inert carbide crystals. The black deposit has a good affinity for the steel surface, including the surface of the inert carbide crystals, and provides an overall black color which complements and intensifies the degree of blackness produced by the oxidation reaction on the elemental iron.

Furthermore, while it requires from 15 to 30 or more minutes for the conventional blackening bath, in the absence of the molybdic acid or salt, to produce a blackening effect upon the elemental iron present at the surface of hardened steel, the addition of the molybdic acid or salt reduces the time period by about 20% and, more importantly, provides a composition which blackens the entire surface including the inert carbide crystals.

The ingredients of the present compositions are quite critical and care must be taken to exclude materials which react with the ingredients to destroy their required reactivity. For instance, materials such as dichromate salts are frequently used in stainless steel blackening compositions to impart or restore corrosion resistance. However such latter compositions are free of alkali metal nitrites because these chemicals are coreactive and also because an adequate blackening of stainless steel can be accomplished in the absence of the alkali metal nitrite. However, in the case of hardened steels, the nitrite salt is an essential ingredient since it produces a deeper black oxidation reaction with the elemental iron present therein. Therefore the present compositions must be free of dichromate salts.

As discussed supra, hardened steels are produced by heating the steel to a critical elevated temperature and then cooling at a rate which fosters the formation of hard martensite rather than ferrite or pearlite. Most commonly such cooling is accomplished by quenching the hot steel in an oil bath. The hardened steel is then prepared for blackening by degreasing it in a solvent bath to remove grease, oil and dirt, then pickling it in a hydrochloric acid bath and finally rinsing with water.

The blackening bath is prepared by adding from about 2 to 10 pounds, preferably from 4 to 8 pounds and most preferably about 6 pounds, by weight of the blackening salt composition to enough water to produce each gallon of the bath and then heating to the boiling temperature which will be between about 255 F and 325 F. The hardened steel to be blackened is immersed in the bath for a period of from about 10 to 25 minutes, more or less, depending upon the concentration of the bath and the type of hardened steel being treated. Thereafter the steel piece is removed from the bath, rinsed in cold water and either dried or dipped in lngredients Parts by Weight Sodium hydroxide 82 Sodium nitrate 5 Sodium nitrite l3 2mercaptoimidazoline 0.25 Wetting agcnt 0.005 Sulfonatcd dye salt 0.005 Sodium molybdatc 0.25

Example 2 Ingredients Parts by Weight Sodium hydroxide 70 Sodium nitrate Sodium nitrite Z-mcrcaptobenzothiazole 0.3 Wetting agent 0.0l Sulfonated dye salt 0.01 Molybdic acid 0.3

The foregoing compositions consist essentially of water-soluble solids which are homogeneously mixed together to form stable blackening salt compositions capable of quick and complete dissolution in hot water. Preferred wetting agents are those set forth in my aforementioned US. Pat. No. 2,960,420, particularly the alkyl naphthalene sodium sulfonate available under the trademark Petro AA. Suitable sulfonated dye salts are the metal salts, preferably sodium salts, of sulfo nated dyes from the group consisting of diphenyl-disazo-bis-8-amino-l-naphthol-3, 6 disulfonic acid, 8- anilino-S-(p-hydroxy anilino)-l-naphthalene sulfonic acid and the sulfonated anthraquinone vat dyes, as disclosed in my aforementioned US. Pat. No. 2,960,420.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. A dry composition suitable for use in aqueous solution for the blackening of hardened steel, consisting essentially of from 50 to 90 parts by weight of an alkali metal hydroxide, from 0.1 to 50 parts by weight of an alkali metal nitrate, from 0.1 to 50 parts by weight of an alkali metal nitrite, from 0.00] to 20 parts by weight of a cyclic sulphurcontaining compound from the group consisting of 2-mercaptobenzothiazole, benzothiazyl disulphide and 2-mercaptoimidazoline, from 0 to 20 parts by weight of a wetting agent, from O to 20 parts by weight of a metal salt of a sulfonated dye and from 0.001 to 20 parts by weight of a material from the group consisting of molybdic acid and alkali metal molybdate salts.

2. A composition according to claim 1 comprising sodium hydroxide, sodium nitrate, sodium nitrite, 2- mercaptoimidazoline, wetting agent and sodium molybdate.

3. An aqueous solution of the composition according to claim 1 comprising from 2 to 10 pounds by weight of said dry composition dissolved in enough water to produce each gallon of said aqueous solution.

4. An aqueous solution according to claim 3 comprising from 4 to 8 pounds by weight of said dry composition per gallon of said aqueous solution.

5. Process for the blackening of steel, particularly hardened steel, comprising the step of immersing the steel for a period of from about 10 to 25 minutes in an aqueous bath heated to a temperature of from about 25 5 to 325F and consisting essentially of an aqueous solution of from to 90 parts by weight of an alkali metal hydroxide, from 0.1 to 50 parts by weight of an alkali metal nitrate, from 0.1 to 50 parts by weight of an alkali metal nitrite, from 0.001 to 20 parts by weight of a cyclic sulphur-containing compound from the group consisting of 2-mercaptobenzothiazole, benzothiazyl disulphide and 2-mercaptoimidazoline, from 0 to 20 parts by weight of a wetting agent, from 0 to 20 parts by weight of a metal salt of a sulfonated dye and from 0.001 to 20 parts by weight of a material from the group consisting of molybdic acid and alkali metal m0- lybdate salts.

6. Process according to claim 5 in which the steel is degreased and pickled prior to immersion in the blackening bath.

7. Process according to claim 5 in which the blackened steel is coated with a protective coating after removal from the blackening bath.

8. Process according to claim 5 in which said aqueous bath comprises from about 4 to 8 pounds of the listed ingredients per gallon of the aqueous bath.

9. Process according to claim 5 in which said aqueous per gallon of the aqueous bath. 

1. A dry composition suitable for use in aqueous solution for the blackening of hardened steel, consisting essentially of from 50 to 90 parts by weight of an alkali metal hydroxide, from 0.1 to 50 parts by weight of an alkali metal nitrate, from 0.1 to 50 parts by weight of an alkali metal nitrite, from 0.001 to 20 parts by weight of a cyclic sulphurcontaining compound from the group consisting of 2-mercaptobenzothiazole, benzothiazyl disulphide and 2-mercaptoimidazoline, from 0 to 20 parts by weight of a wetting agent, from 0 to 20 parts by weight of a metal salt of a sulfonated dye and from 0.001 to 20 parts by weight of a material from the group consisting of molybdic acid and alkali metal molybdate salts.
 2. A composition according to claim 1 comprising sodium hydroxide, sodium nitrate, sodium nitrite, 2-mercaptoimidazoline, wetting agent and sodium molybdate.
 3. An aqueous solution of the composition according to claim 1 comprising from 2 to 10 pounds by weight of said dry composition dissolved in enough water to produce each gallon of said aqueous solution.
 4. An aqueous solution according to claim 3 comprising from 4 to 8 pounds by weight of said dry composition per gallon of said aqueous solution.
 5. PROCESS FOR THE BLACKENING OF STEEL, PARTICULARLY HARDENED STEEL, COMPRISING THE STEP OF IMMERSING THE STEEL FOR A PERIOD OF FROM ABOUT 10 TO 25 MINUTES IN AN AQUEOUS BATH HEATED TO A TEMPEATURE OF FROM ABOUT 255* TO 325*F AND CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF FROM 50 TO 90 PARTS BY WEIGHT OF AN ALKALI METAL HYDROXIDE, FROM 0.1 TO 50 PARTS BY WEIGHT OF AN ALKALI METAL NITRATE, FROM 0.1 TO 50 PARTS BY WEIGHT OF AN ALKYL METAL NITRITE, FROM 0.001 TO 20 PARTS BY WEIGHT OF A CYCLIC SULPHUR-CONTAINING COMPOUND FROM THE GROUP CONSISTING OF 2-MERCAPTOBENZOTHIAZOLE, BENZOTHIAZYL DISULPHIDE AND 2-MERCAPTOIMIDAZOLINE, FROM 0 TO 20 PARTS BY WEIGHT OF A WETTING AGENT, FROM 0 TO 20 PARTS BY WEIGHT OF A METAL SALT OF A SULFONATED DYE AND FROM 0.001 TO 20 PARTS BY WEIGHT OF A MATERIAL FROM THE GROUP CONSISTIN OF MOLYBDIC ACID AND ALKALI METAL MOLYDBATE SALTS.
 6. Process according to claim 5 in which the steel is degreased and pickled prior to immersion in the blackening bath.
 7. Process according to claim 5 in which the blackened steel is coated with a protective coating after removal from the blackening bath.
 8. Process according to claim 5 in which said aqueous bath comprises from about 4 to 8 pounds of the listed ingredients per gallon of the aqueous bath.
 9. Process according to claim 5 in which said aqueous bath comprises about 6 pounds of the listed ingredients per gallon of the aqueous bath. 